1. Field of the Invention
The present invention relates to page-based optical data storage and, in particular, to a sensor and method for providing increased transfer rates in page-based optical data storage.
2. Background Art
Traditional optical data storage technologies (e.g., CD and DVD) lag behind traditional magnetic data storage technologies (e.g., hard drives and tape) in transfer rates. Page-based optical data storage and retrieval offers the advantage of parallel optical data processing and may generally enable higher data transfer rates.
In page-based optical data storage, a page of data is read by imaging a two-dimensional data page onto a two-dimensional detector. The detector detects the image, and the image is subsequently processed to decode the data contained therein. In general, the detector comprises an active (i.e., absorbing) layer for detecting the image of the two-dimensional data page.
Page-based optical data storage technologies and, in particular, holographic data storage (HDS), have benefited greatly from the recent development of low cost complementary metal oxide semiconductor (CMOS) active pixel digital image sensors (also known as APS detectors), such as the ones available today in high resolution digital cameras. These sensors, compared to traditional charge-coupled device (CCD) detectors, present the advantages of lower unit cost (due to the simpler design and manufacturing) and design flexibility provided by the CMOS process, allowing more functionality to be included in the detector chip. However, these sensors have invariably been optimized for the high dynamic range and relatively low transfer rates required by consumer imaging and machine vision applications, not for data storage.
Recent efforts to increase the transfer rate have focused on improvements to the back-end (i.e., off-chip) electronic transfer and processing architecture of the detector. Major advances include the implementation of increasingly complex multi-element active pixel structures having multiple transistors and memory elements for improving signal to noise ratio and enabling parallel integration and readout. The incorporation of multiple parallel output amplifiers has increased the electronic output from the image sensor. While such techniques have yielded substantial improvements in the frame transfer rates, they do not improve the optical properties of the sensor and sometimes contribute to their degradation. For example, the integration of a greater number of electronic circuitry tends to reduce the photoactive area in each pixel, otherwise termed its fill factor, and thus contributes to reducing optical detection efficiency. Furthermore, there exists a general trade-off between bandwidth and sensitivity of a photo sensor, both being dependent on the thickness of the active layer of the detector. In particular, as the thickness of the active layer decreases, the detector bandwidth increases but the detector sensitivity and also the detector signal to noise ratio generally decrease.
Background information may be found in Geoffrey W. Burr, Holographic Storage, Encyclopedia of Optical Engineering, Marcel Dekker, Inc., 2003; Geoffrey W. Burr et al., Modulation Coding For Pixel-Matched Holographic Data Storage, Optical Letters, Vol. 22, No. 9, Optical Society Of America, 1997; Nicolas Blanc, CCD versus CMOS—has CCD imaging come to an end?, Photogrammetric Week ‘01’, D. Fritsch and R. Spiller Eds. Wichmann Verlag, Heidelberg, 2001; and Jean Schleipen et al., Optical Heads, Encyclopedia Of Optical Engineering, Marcel Dekker, Inc., 2003.
Accordingly, it may be desirable to have a sensor (i.e., detector) and/or method for providing greater sensitivity without requiring an increase in thickness of a corresponding photoactive area.